1. Field of the Invention
The present invention relates to a magnetic information transfer apparatus for transferring a magnetically recorded information on a master magnetic recording medium to a slave magnetic recording medium.
2. Description of the Prior Art
As a magnetic information transfer apparatus for transferring a magnetic recording information recorded on a master magnetic recording medium (referred to as "master tape" hereinafter) to another magnetic recording medium having no information recorded (referred to as "slave tape" hereinafter), thermal transfer type and magnetic field transfer type systems have been proposed. Among others, the magnetic field transfer type apparatus, in which a master tape and a slave tape are passed through a bias magnetic field for information transfer in a direction orthogonal to the bias magnetic field, with magnetic surfaces thereof being in an intimate contact with each other, to transfer the magnetic information on the master tape to the slave tape, is advantageous compared with a case where a copy tape is obtained according to the so-called 1:1 dubbing system in that it is possible to obtain a copy tape at higher speed.
FIG. 1 shows an example of a bias magnetic field generator of a conventional magnetic field transfer type apparatus. In FIG. 1, a reference numeral 1 depicts a main head base, 2 a sub-head base, 3 a head holder and 4 a magnetic head for generating a bias magnetic field to be used to transfer a magnetic information. The magnetic head 4 takes in the form of the so-called C type head having a magnetic gap 4a. A reference numeral 5 depicts a drum of soft magnetic material which is rotated by a rotary shaft 6.
The magnetic head 4 is provided on a structural portion including the main head base 1 and the sub-head base 2 such that the magnetic gap 4a thereof is positioned with respect to an outer peripheral surface of the drum 5 with a minute distance thereto. Side face portions 1a and 2a of the main and sub head bases 1 and 2 are faced to the outer peripheral surface of the drum 5 with minute distances with respect thereto.
A master tape 7 and a slave tape 8 having magnetic surfaces in intimate contact with each other are held in pressure contact with the outer peripheral surface of the drum 5 by air jetted from orifices provided in the main and sub head bases 1 and 2 and run along with rotation of the drum while being subjected to magnetic flux extending from the magnetic gap 4a of the magnetic head 4 toward the drum 5.
When the drum 5 is not positioned in the vicinity of the magnetic gap 4a of the head 4, a horizontal component of magnetic flux leakage from the magnetic gap 4a becomes major and, when the drum 5 is arranged in proximity to the magnetic gap 4a, a vertical component of the magnetic flux leakage from the gap 4a becomes major.
FIG. 2 is a graph showing a leakage magnetic field distribution in the magnetic gap 4a of the magnetic head 4 shown in FIG. 1, which is obtained by a model experiment. As is clear from FIG. 2, a vertical component Hy (Hy') of the magnetic field leakage in the gap 4a becomes symmetrical about a center line of the gap 4a with peaks being on both sides of the center line of the gap.
In the magnetic field strength distribution shown in FIG. 2, a portion thereof which is inherently necessary as a bias magnetic field for transferring an information recorded on the master tape to the slave tape is only a descending slope portion of the right side peak thereof. The left side distribution is an unnecessary portion and the left side peak portion may contribute to demagnetization of the master tape 7 during it passes therethrough.
In the field distribution shown in FIG. 2, the vertical magnetic field strength curve Hy and the horizontal magnetic field strength curve Hx shown by solid lines are obtained when nozzle members NB1 and NB2 provided in the vicinity of the magnetic had 4 for jetting air to pressure-contact the master tape 7 and the slave tape 8 with the outer peripheral surface of the drum 5 are of non-magnetic and electrically non-conductive material such as ceramics. The vertical magnetic field strength curve Hy' and the horizontal magnetic field strength curve Hx' shown by dotted lines are obtained when the nozzle member NB1 provided on the left side of the head 4 is of non-magnetic and electrically non-conductive material and the right side nozzle member NB2 is of non-magnetic and electrically conductive material such as aluminum alloy or stainless steel, etc.
In the distribution curves shown in FIG. 2, the descending portion of the right side peak of the vertical magnetic strength curves Hy' is steeper than that of the curve Hy and the descending portion of the horizontal curve Hx' is also steeper than that of the curve Hx. This is because that a reverse magnetic field which cancels magnetic line of force passed through the metal nozzle members provided in the vicinity of the magnetic head 4 is generated with which eddy current flows through the metal nozzles to reduce magnetic field strength in the skirt portions of the descending portions on the right side of the center of the gap 4a.
When the slope of the skirt portion of the right side portion of the magnetic field strength curve becomes steep, a region which is subjected to the bias magnetic field becomes narrow. Therefore, a magnetic transfer region in which the master tape 7 and the slave tape 8 are in pressure-contact with the outer peripheral surface of the drum 5 without vibration by means of air jetted from the nozzles becomes narrow advantageously. However, it is necessary to set frequency of current for generating the bias magnetic field at high value such that the bias magnetic field alternates a predetermined number of times or more within such narrow transfer region.
In the structure in which the metal nozzle members are arranged in the vicinity of the magnetic head 4, eddy current flows in the metal nozzle member as mentioned previously and the slope of the skirt of the descending portion of the right side peak of the magnetic field strength distribution with respect to the center line of the magnetic gap 4a of the head 4 becomes steep. However, the efficiency of the bias magnetic field generator is necessarily lowered due to such eddy current. Further, due to increase of magnetic core loss caused by the use of high frequency current, heat generation becomes considerable. In addition, due to high frequency current, a drive circuit therefor becomes expensive.
For the drum 5 of soft magnetic material which is rotated by the rotary shaft 6 driven by driving power transmitted from a power source, not shown, it rotates with some eccentricity dependent on manufacturing preciseness of bearings supporting the drum and circularity of the drum, etc., and the bias magnetic field varies with variation of distance between the magnetic head and the outer peripheral surface of the drum due to the eccentric rotation of the drum. Thus, it is difficult to obtain a cop tape of acceptable quality.
Further, in the bias magnetic field generator shown in FIG. 1, passage for compressed air is provided in a side surface of the C shaped magnetic head. Since, in order to pressure-contact the master tape and the slave tape with the relatively wide outer peripheral surface of the drum in view of magnetic saturation of the core, a large amount of compressed air is necessary.